The present invention relates to hearing aid systems, and more particularly to a hearing aid system that combines an implantable cochlear stimulator system, including a cochlear electrode array, with a cochlear acoustic transducer, coupled to an implantable or other microphone. Such system relies primarily on the cochlear stimulator portion for sensing high frequency sounds, and relies primarily on normal hearing processes, augmented by the cochlear acoustic transducer, for sensing lower frequency sounds. Such hearing aid system utilizes a short cochlear electrode array, e.g., of the type described in applicant's copending patent application, Ser. No. 60/134,290, filed May 14, 1999, entitled "Electrode Array for Hybrid Cochlear Stimulator", into which an acoustic transducer has been integrated.
Hearing loss is generally of two types: conductive and sensorineural. Of these, conductive hearing loss occurs where the normal mechanical pathways for sound to reach the hair cells in the cochlea are impeded, for example, by damage to the ossicles. Conductive hearing loss may often be helped by use of conventional hearing aids, which amplify sound so that acoustic information does reach the cochlea and the hair cells. Some types of conductive hearing loss are also amenable to alleviation by surgical procedures.
Sensorineural hearing loss, on the other hand, results due to the absence or the destruction of the hair cells in the cochlea which are needed to transduce acoustic signals into auditory nerve impulses. Persons who suffer from sensorineural hearing loss are unable to derive any benefit from conventional hearing aid systems, no matter how loud the acoustic stimulus is made, because their mechanisms for transducing sound energy into auditory nerve impulses have been damaged. Thus, in the absence of properly functioning hair cells, there is no way auditory nerve impulses can be generated directly from sounds.
To overcome sensorineural deafness, there have been developed numerous cochlear implant systems--or cochlear prosthesis--which seek to bypass the hair cells in the cochlea by presenting electrical stimuli directly to the ganglia of the auditory nerve located adjacent the modiolar wall of the cochlea. When triggered, the ganglia, also referred to as ganglion cells, send nerve impulses to the brain via the auditory nerve, leading to the perception of sound in the brain, and an at least partial restoration of hearing function. The common denominator in these cochlear prosthesis systems has been the implantation into the cochlea of electrodes which are responsive to a suitable external source of electrical stimuli and which are intended to transmit those stimuli to the ganglion cells, and thereby to the auditory nerve fibers.
As people age, they frequently experience progressive hearing loss. Usually this loss is more prevalent and more severe at higher frequencies. Thus, it is estimated that a large segment of the hearing-impaired population exhibit sensorineural hearing loss relative to high frequency sounds, but maintain the ability to transduce middle-to-lower frequency sounds through functioning hair cells.
The usual method to restore this high frequency hearing loss is by using a hearing aid that increases the amplitude of the acoustic energy applied to the tympani membrane. Although effective, this approach does not provide the same level of restoration to high frequencies as it does to lower frequencies. Also, the increase in acoustic amplitudes used in this method can ofttimes further degrade residual hearing, resulting in a further decrease in the ability to hear the higher frequencies.
It is thus evident that there is a need for a"hybrid" cochlear stimulation system that electrically stimulates only the ganglion cells responsible for sensing higher frequency sounds, while allowing or permitting the normal hearing process (e.g., activation of hair cells through wave motion of the fluid within the cochlea) to function for the purpose of sensing lower-to-middle frequency sounds.
A cochlear prosthesis operates by direct electrical stimulation of the auditory nerve cells, bypassing the defective cochlear hair cells that normally transduce acoustic energy into electrical activity in such nerve cells. Because the ganglion cells responsible for sensing higher frequency sounds are all generally located in or near the basal end of the cochlea (the end of the cochlea nearest the round window), a hybrid cochlear stimulation system thus requires an electrode array that can be inserted within the cochlea a sufficient depth to be near such cells, but which also does not block or significantly interfere with the normal functioning of the cochlea for hair cells located deeper within the cochlea. Thus, there is a need for such an electrode array that may be used with an implantable cochlear stimulator hearing system.
Because any electrode array inserted into the cochlea is likely to block the cochlea to a certain extent, and thereby interfere (even if only minimally) with the normal functioning of the hair cells located deeper within the cochlea, there is also a need to augment, or to restore, the normal hearing pathways (i.e., the fluid path) to those hair cells located deep within the cochlea.